Power consumption and efficiency We're trying something a little different with power consumption. Our Extech 380803 power meter has the ability to log data, so we can capture power use over a span of time. The meter reads power use at the wall socket, so it incorporates power use from the entire systemthe CPU, motherboard, memory, video card, hard drives, and anything else plugged into the power supply unit. (We plugged the computer monitor and speakers into a separate outlet, though.) We measured how each of our test systems used power during a roughly one-minute period, during which time we executed Cinebench's multithreaded rendering test. All of the systems had their power management features (such as SpeedStep and Cool'n'Quiet) enabled during these tests.

You'll notice that I've not included the Athlon 64 FX-72 here. That's because our "simulated" FX-72 CPUs are underclocked versions of faster processors, and we've not been able to get Cool'n'Quiet power-saving tech to work when CPU multiplier control is in use. I have included test results for genuine Athlon 64 X2 4400+ and 5600+ chips, as promised in our last CPU roundup.

I have included our simulated Core 2 Duo E6600 and E6700, because SpeedStep works fine on the D975XBX2 motherboard alongside underclocking. The simulated processors' voltage may not be exactly the same as what you'd find on many retail E6600s and E6700s. However, voltage and power use can vary from one chip to the next, since Intel sets voltage individually on each chip at the factory.

The differences between the CPUs are immediately obvious by looking at these plots of the raw data. We can slice up the data in various ways in order to better understand them, though. We'll start with a look at idle power, taken from the trailing edge of our test period, after all CPUs have completed the render.

There's no hiding the presence of a second chip on the QX6800's package, even at idle. It simply draws more power than the dual-core Core 2 CPUs. That said, our QX6800 draws six fewer watts at idle than our Core 2 Extreme QX6700.

Next, we can look at peak power draw by taking an average from the five-second span from 10 to 15 seconds into our test period, during which the processors were rendering.

Impressively, the QX6800 pulls only 2W more juice under load than its predecessorand the top Intel quad-core system draws less power than the Athlon 64 X2 6000+-based rig.

Another way to gauge power efficiency is to look at total energy use over our time span. This method takes into account power use both during the render and during the idle time. We can express the result in terms of watt-seconds, also known as joules.

The QX6800 finishes middle of the pack in energy use over the test time period, ahead of the QX6700. That's largely because it finishes the render first and thus drops back down to idle power levels sooner. Power use on the Quad FX systems is... embarrassing.

We can quantify efficiency even better by considering the amount of energy used to render the scene. Since the different systems completed the render at different speeds, we've isolated the render period for each system. We've chosen to identify the end of the render as the point where power use begins to drop from its steady peak. There seems to be some disk paging going on after that, but we don't want to include that more variable activity in our render period.

We've computed the amount of energy used by each system to render the scene. This method should account for both power use and, to some degree, performance, because shorter render times may lead to less energy consumption.

This result encapsulates nicely the progress Intel has made on power efficiency since our QX6700 was manufactured. The QX6800 draws about the same amount of power while running at a higher clock frequency, allowing it to complete the task using less total energy than the QX6700. What's more, the Intel Kentsfield quad-core processors are the most energy efficient in this test, since this workload can be broken into four parallel threads with ease.